1 Field of the Invention
This invention relates in general to diverters and blowout preventer systems for drilling rigs. In particular, the invention relates to a system adapted for alternative use as a diverter or a blowout preventer.
2 Description of the Prior Art
Diverter systems are known for drilling rigs in which a diverter element is provided in the support housing attached to the support beams beneath the drilling rig rotary table. Such diverter systems have provided a vent line and a flow line in the permanent housing beneath the rotary table. Such systems have required external valve systems in the vent line to open the fluid system to the vent line when the diverter is closed so that fluid flow may be directed away from the drilling rig. Such diverter systems have been provided not only for floating vessel drilling rigs, but also for bottom supported offshore drilling rigs and for land rigs.
Fatal and costly accidents have resulted from the complexity of the prior art diverter systems described above. Typical prior art diverter systems have included an annulus closing device, external vent and flow line valves, actuators, limit switches and sequenced controls. This complicated valving and piping of the prior art has been further complicated by the inherent risks of manipulating loose packer inserts into the diverter itself. The complexity of the prior art systems has invited a variety of human error and equipment malfunctions.
One problem with the prior art systems has involved the use of external valving in the diverter system. Valves which are external to the diverter unit not only add clutter to the diverter system and the rig configuration, they have also required multiple control functions which are required to operate correctly. For example, prior art diverter system valves have required an actuating pressure signal that is regulated to a discrete pressure level different from the operating pressure level of the diverter unit. The need for separate and different control functions executed in only one safe sequence has required separate pressure regulators and connecting functional components that are in different locations on the underside of the rig floor. Such a requirement has invited mistakes and malfunctions.
In addition to the problem of multiple control functions, there has existed problems with mistakenly crossed hydraulic connections in prior art diverter systems. Misconnection of control lines can cause a valve to be closed when it should be open or vice versa potentially resulting in an explosion in the diverter system or breach of the casing.
Another problem of the prior art diverter systems has been exposure to the working environment of delicate parts such as hydraulic tubing and fittings, limit switches, mechanical linkages and valve actuators. Such exposure has in the past caused occasional breakage and damage to such parts. Delicate parts can be damaged or broken by impact with heavy equipment, use as steps or handholds by working personnel, or vibrations induced by running equipment. System malfunctions which result from such damage can be catastrophic.
Another hazard of prior art diverter systems has been the result of vent line blockage because the vent valve has been remote from the diverter unit itself. A stagnant space has existed at a critical location in the vent line. Build up of ice or other solids and/or caking of mud in such a dead space may cause the critically important vent line to be choked off. A restricted or shut-off vent line may cause a dangerous pressure increase while being called upon to divert.
Still another problem of prior art diverter systems has involved the use of component sources from a number of different manufacturers. The annulus closing device, vent and flow line valves, actuators, sequencing devices and control system components have typically each been provided by a different manufacturer. Rig operating personnel are usually burdened with devising the vent line valve circuit interconnecting the components (which are often widely physically separated when installed) and stocking a varied assortment of spare parts using extraordinary caution to avoid misconnections and keeping a number of rig personnel trained to operate and maintain a diverse assortment of complicated components.
Some prior art diverter systems for bottom supported rigs have included the use of a high pressure external valve in the vent line to control the diverting function. Closure of such a valve has enabled the diverter to be converted to a blowout preventer after sufficient casing pressure integrity has been established during drilling operations. However, if this valve should inadvertently be closed during an attempt to divert, breach of the casing or explosion of the diverter system could threaten the safety of the rig itself.
Still another problem of prior art diverter systems has been the result of valve mismatch. While many different types of valves have been used in diverter systems, there has been no single valve that is especially well suited to the particular application of a diverter system. Selection of the type, size and rating of such valves has been a vexing puzzle for designers of rig valve systems which has been required to be solved usually when a new drilling rig is being built.
Perhaps the most destructive problem of the prior art diverter systems has been the inherent risk of pressure testing in situ. Pressure testing of prior diverter systems has been accomplished by overriding the safety sequencing in the valves so that the vent line valve is closed simultaneously with closure of the annulus. Such problem is inherent not only in the packer insert type diverter systems, but also the annular blowout preventer/spool type diverter systems. Disastrous results have been experienced when the safety overriding mechanism has been unintentionally left in place when testing is completed and drilling is resumed.
Still another problem in the prior art is the effects of the initial flow of fluid and solids in the event of a shallow gas kick. Forces from this initial flow create high pressures on the drilling rig including the blowout preventer seals and huge reaction forces on supports for the vent lines.